Articulated hold-down anchor device for the embedded cables of a prestressed concrete girder

ABSTRACT

An articulated hold-down anchor device which is designed for use in connection with the embedded cables of a prestressed concrete girder and embodies a frame which supports the usual cable holddown rollers, is of a free floating nature so that the upward pull thereon is entirely axial, and is characterized by the fact that it and also the anchor bolt which supports it are devoid of bending stresses. A novel swivel connection at the base of the frame allows the cable immediately thereabove to lie within the prescribed two-inch minimum distance from the base of the concrete form in which the girder is produced.

United States Patent 1191 Colado et a1.

[ Dec. 17, 1974 [75] Inventors: Joseph J. Colado, Addison, 111.;

Norman G. Turner, Lake Bluff, I11.

[73 Assignee: Superior Concrete Accessories, Inc.,

Des Plaines, Ill.

[22] Filed: May 17, 1973 [21] Appl. No.: 361,216

[52] US. Cl 52/226, 248/58, 249/94, 425/1 1 l 51 1m. (:1. E04c 3/10 [58] Field of Search 52/225, 226; 248/59, 58, 248/63, 61; 249/93, 94; 425/111 [56] References Cited UNITED STATES PATENTS 591,249 10/1897 Emery 248/58 1,604,590 10/1926 McNulty 248/59 3,003,217 10/1961 Gerwick. .lri 52/225 3,006,114 10/1961 Hillberg 52/225 3,091,897 6/1963 Hillberg .1 52/225 3,230,678 1/1966 Eriksson et a1. 52/226 Primary ExaminerAlfred C. Perham Attorney, Agent, or FirmNorman H. Gerlach [5 7 ABSTRACT An articulated hold-down anchor device which is designed for use in connection with the embedded cables of a prestressed concrete girder and embodies a frame which supports the usual cable hold-down rollers, is of a free floating nature so that the upward pull thereon is entirely axial, and is characterized by the fact that it and also the anchor bolt which supports it are devoid of bending stresses. A novel swivel connection at the base of the frame allows the cable immediately thereabove to lie within the prescribed two-inch minimum distance from the base of the concrete form in which the girder is produced.

1 Claim, 5 Drawing Figures PATENTEB BEE] H974 1854258 ARTICULATED HOLD-DOWN ANCHOR DEVICE FOR THE EMBEDDED CABLES OF A PRESTRESSED CONCRETE GIRDER The present invention relates to a hold-down anchor by means of which the tensioning cables of a prestressed concrete girder are held taut and in their proper relative positions within the girder form during initial pouring and subsequent hardening of the girderforming concrete to the end that, when the tension in the cables is relieved after the concrete has become set and the projecting ends of the cable have been severed from the ends of the girder, the concrete of the girder will remain in a so-called prestressed condition, thus providing materially greater strength than a conventional girder which is not prestressed in this manner.

In the construction of a prestressed concrete girder of the aforementioned type, the tensioning cables are ordinarily strung longitudinally through the girder form in different but generally longitudinally extending directions. For example, in a prestressed concrete beam type girder having an upper head portion, a lower base portion, and a narrow interconnecting web portion between the two portions, certain of the cables are strung through the girder form so that they extend inwardly from the opposite ends of the girder in the head portion thereof and then are inclined downwardly so that their central or intermediate portions pass through the narrow web portion of the girder and enter the base portion, after which they extend longitudinally in straight line fashion through the base portion of the girder in the medial region of the latter. The cables are thus caused to make a dip, so to speak, as they pass through the girder from one end to the other so that the end regions of the cables are embedded in the girder head portion while the medial regions are embedded for the most part in the girder base portion.

in order to impart to the cables the aforementioned directional characteristics, it is customary to utilize a plurality of hold-down anchor devices which are bolted to and extend upwardly from the base member of the concrete form, such devices being in the form of rigid, vertically elongated frames embodying vertically spaced hold-down rollers or other cable-engaging members beneath which the series of tensioning cables extend. These frames extend substantially vertically and, in connection with the earlier anchor devices, they were fixed with respect to the base member of the form. It was found that with these early devices, the lateral upward pull which is applied thereto as a result of tensioning of the cables has a tendency to place on the frames of the devices a vertorial force which deviates from the vertical. The magnitude of this force is appreciable when it is considered that the upward thrust on each hold-down roller is of the order of 3,000 pounds. Such a force, directed at an angle, frequently results in bending of the anchor bolts which hold the frames, such bending serving to weaken the bolts to such an extent that they give way and release the cables.

in an effort to relieve the roller-supporting frames of such hold-down anchor devices of such bending stresses, it has recently been proposed that the frames be permitted to yield to the unequal lateral force which is applied thereto due to direction changes in the cables. This is done by establishing pivoted swivel-type connections between the lower ends of the frames and the base member of the girder form, in which case the frames will yield so that the angle of incidence with respect to any given roller will be equal to the angle of emergence with the net result that the frames will be placed in longitudinal tension with no bending forces being applied thereto. it has been found that, with such pivoted swivel connections, relatively long anchor bolts are required for frame-attachment purposes and, furthermore, relatively long, socketed, supporting columns must be provided for the bolts. The use of such bolts and socket members necessitates placing the lowermost hold-down rollers or shafts an appreciable distance above the girder form base, well in excess of the prescribed two-inch minimum distance, and thus, in the finished concrete girder, the extreme lower portion of the girder remains unstressed.

The present invention is designed as an improvement over present-day as well as previously developed holddown anchors of the swinging or swivelled frame type in that it affords a novel swivel connection for the lower end of the roller-carrying frame whereby such minimum two-inch requirement for the height of the lowermost roller may be maintained without sacrificing the strength of the connection as a whole. Accordingly, the present invention contemplates that, instead of utilizing a vertically elongated support column for receiving the associated upstanding anchor bolt on the base member of the concrete girder form, a relatively large diameter cylindrical pivot shaft is employed and it extends across the lower region of the frame and is provided with a vertically extending threaded bore through which the anchor bolt extends upwards. The extreme lower end of the roller-supporting frame overhangs the swivel shaft so that when the hold-down anchor device is operative'ly secured to the base member of the girder form, the lower end of the frame is disposed but a short distance above such base member, thereby bringing the lowermost roller in the vertical series of rollers within the prescribed two-inch minimum requirement. Although when the anchor bolt is tightened, the lower end of the frame may seat on or slightly engage the base member of the girder form, the efficiency of the swivel connection for the purpose intended is not appreciably impaired inasmuch as the tremendous upward pull of the cables on the frame will easily slide or rock the lower end of the frame over the plywood base of the girder form the few thousandths of an inch which is necessary to accommodate the slight angular displacement of the frame away from a true vertical position. The end portions of the cylindrical pivot shaft are of re.- duced diameter and form trunnions which extend through aligned holes in the lower portions of the frame sides and constitute the pivotal connection between the shaft and the frame of the device.

The provision of a hold-down anchor device such as has been briefly outlined above, and possessing the stated advantages, constitutes the principal object of the present invention.

Other objects and advantages of the invention, not at this time enumerated, will readily suggest themselves as the following description ensues.

The invention consists of the several novel features which are hereinafter set forth and are more particularly defined by the claim at the conclusion hereof.

in the accompanying single sheet of drawings forming a part of this specification, one illustrative embodiment of the invention is shown.

In these drawings:

FIG. 1 is a fragmentary sectional view taken substantially centrally and longitudinally through a concrete girder form, showing a pair of hold-down anchor devices embodying the present invention operatively installed therein;

FIG. 2 is an enlarged perspective view of the swivel shaft which is employed in connection with the invention;

FIG. 3 is an enlarged vertical sectional view taken substantially on the line 3-3 of FIG. 1 and showing the concrete girder form without concrete therein;

FIG. 4 is a horizontal sectional view taken substantially on the line 44 of FIG. 3; and

FIG. 5 is a fragmentary exploded perspective view of the hold-down anchor device of FIG. 3.

Referring now to the drawings in detail and in particular to FIG. 1, a prestressed concrete girder utilizing a plurality of hold-down anchor devices embodying the present invention is fragmentarily illustrated and is designated in its entirety by the reference numeral 10. The girder is shown as being in position within a form which is initially prepared to receive the concrete of the girder during a pouring operation. Only the fiat bottom or base member 12 of the form is shown and only two of the hold-down anchor devices appear in FIG. 1. For purposes of description herein, it may be assumed that the girder is in the form of an l-beam and that the section represented by this view is taken longitudinally and centrally through the web and lower base portion of the girder. For a full disclosure of a prestressed concrete girder having the configuration of an l-beam and utilizing hold-down anchor devices in the web and lower base portion thereof, reference may be had to U.S. Pat. No. 3,006,014, granted on Oct. 31, 1961 and entitled HOLD-DOWN ANCHOR DEVICE FOR THE EMBEDDED CABLES OF PRESTRESSED CONCRETE GIRDERS.

In the exemplary illustration of FIG. 1, a series of tensioning cables 14 in tier form extends generally in a longitudinal direction completely through the girder 10 from end to end and assume various directional positions within the girder as will be described presently. The cables are preferably comprised of stranded steel wires and they are firmly embedded in the surrounding mass of concrete and are initially positioned in the girder form by means of the aforementioned hold-down anchor devices, each of which is designated in its entirety by the reference numeral 16. Since the two devices are identical, but one need be described in detail.

Each anchor device 16 is in the form of a composite assembly which is designed to accommodate the tensioning cables 14. The illustrated cables 14 extend from the upper region of the girder at one end thereof inwardly and downwardly on an incline through the web portion of the I-beam-type girder and into the base portion of the girder where they are engaged by one of the hold-down anchor devices 16, after which they extend upwardly and outwardly and into the head portion of the girder where they emerge from the latter. Each anchor device 16 embodies a frame 18 including a pair of vertically elongated side plates 20 and 22, each plate comprising a relatively long narrow strip of heavy gauge sheet or bar steel. These side plates are maintained in their spaced apart parallel relationship by means of a plurality of equally and vertically spaced roller-supporting spacer and hold-down assemblies 24 beneath which the associated cables 14 extend, the

function of such assemblies being to hold the individual cables at their desired elevations within the girder form and also to establish the desired direction changes throughout the length of the cables during tensioning of the latter. Various forms of roller-supporting spacer and hold-down assemblies are capable of use in connection with the anchor device of the present invention and no claim is made herein to any novelty associated with the same. There are currently in use various forms of such assemblies, one known form consisting solely of a transverse shaft which extends across the frame from oneside plate to the other side plate. Other forms of hold-down assemblies embody one or more antifriction rollers on shafts extending transversely through the frame. The particular roller-supporting spacer and hold-down assembly which has been selected for illustration herein is of the general type which is shown and described in copending United States patent application Ser. No. 327,8l3, filed on Jan. 29, 1973 in the name of Joseph J. Colado, and entitled ARTICU- LATED HOLD-DOWN ANCHOR DEVICE FOR THE EMBEDDED CABLES OF A PRESTRESSED CONCRETE GIRDER, and reference may be had to such patent application for a full understanding of the manner in which the hold-down assemblies are constructed and how they function for cable hold-down purposes. It is deemed sufficient for descriptive purposes herein to point out that each assembly 24 comprises a horizontal bolt having a shank portion 26 which projects through a pair of aligned holes 28 in the side plates 20 and 22 of the frame 18, a polygonal head 30 at one end of the shank, and a nut 32 at the other end of the shank. A central spacer sleeve 34 is telescopically received on the central portion of the bolt shank 26 and is flanked by a pair of spacer sleeves 36 of larger outside diameter. The nut and bolt head serve to draw the side plates 20 and 22 hard against these sleeves 34 and 36 in order that the frame 18 as a whole is of rigid character. A cable-receiving hold-down roller 38 having an annularly grooved periphery is rotatably mounted on the central spacer sleeve 34 and is of slightly less thickness than the longitudinal extent of such sleeve so that it may have freedom of rotary motion. As shown in FIG. 1, of the drawings, the various cables 14 pass beneath the rollers 38 and, when they are tensioned, they exert an upward thrust of great magnitude of such rollers.

According to the present invention, and in order that no bending stresses shall be applied to the side plates 20 and 22 of the frame 18, a swivel connection is provided for affixing the frame 18 as a whole to the base member 12 of the girder form. This swivel connection embodies a cylindrical horizontally extending pivot shaft 40 (see particularly FIG. 2) and a vertically extending anchor bolt 42. The side plates 20 and 22 are adapted to be held in position above the base member 12 of the concrete girder form by means of the anchor bolt 42, the latter having an upstanding shank portion 44 which is threadedly received in a diametric vertically extending screw-threaded bore 46 in the central portion of the pivot shaft 40. The anchor bolt 42 passes upwardly through an opening or hole 48 in the base member 12 and is provided at the lower end of its shank portion 44 with a bolt head 50 which bears against a washer 52 when the bolt 42 is in its operative position. The cylindrical pivot shaft 40 is provided at its ends with a pair of integral trunnins S4. The latter are coaxial with the central portion of the shaft 40, project outwardly from the opposite circular end faces 56 of the shaft, and extend through respective aligned bearing openings or holes 58 in the side plates and 22 of the frame 18. The diameter of the trunnions is appreciably less than the diameter of the holes 58 in order that the frame 18 is loose with respect to the pivot shaft 40. Annular outwardly extending flanges 60 on the distal ends of the trunnions 54 serve to retain split Tru- Arc or other split snap rings 62 in position on the trunnions. The snap rings 62 are interposed between the annular flanges 60 and the outer faces of the frame side plates 20 and 22 and serve to prevent axial displacement of the pivot shaft 40 with respect to the frame 18. The annular outwardly extending flanges 60 on the distal or outer ends of the trunnions 54 are of slightly less diameter than the holes 58 in the frame side plates 20 and 22 in order to permit the pivot shaft 40 to be assembled with respect to the frame 18. The lower edges of the two frame side plates 20 and 22 are rounded or arcuate as indicated at 64, these arcuate edges being concentric with the axis of the pivot shaft 40 and serving a purpose that will be made clear presently. It is to be noted that the lower ends of the two side plates 20 and 22 overhang the circular end faces 56 of the pivot shaft 40 so that the arcuate end edges 64 of such plates will bear tangentially against the upper surface of the base member 12 of the concrete girder form in the event that, in certain instances, the anchor bolt 42 is tightened to its fullest extent.

In the installation and use of the herein described anchor devices, holes such as those shown at 48 are drilled or otherwise formed in the base member 12 of the concrete girder form in the regions where it is desired to install the anchor devices 16. The anchor bolts 42 are then passed upwardly through the holes and are threadedly received in the threaded bores 46 in the pivot shafts 40. If, as is the case in some instances, the anchor bolts 42 are tightened to their fullest extent, the arcuate lower ends of the side plates 20 and 22 are drawn against the upper surface of the base member 12 of the girder form as clearly shown in FIGS. 1 and 3 and the frames 18 of the anchor devices are caused to stand in substantially vertical erect positions, the particular degree of angularity at this time being not critical. Thereafter, the various cables 14 are threaded through the frames 18 so that each cable underlies an associated hold-down roller 38 of an assembly 24. Upon tensioning of the cables 14 by means of the usual hydraulic tensioning mechanism (not shown), the cables will be drawn upwardly against their associated rollers 38 and,

as the tension is progressively applied, the rollers will turn on the central spacer sleeves 34 and afford rolling tractional support to the cables.

Because of the fact that each frame 18 is pivotally connected at its lower end to the base member 12 of the concrete girder form, the frame will seek a position of equilibrium wherein the upward thrust of the cables 14 on the rollers 38 will cause the frame to bisect the deviation angle which is established in each cable at its region of contact with the associated roller. Stated otherwise, the resultant angle of incidence of each cable will equal the angle of emergence so that the upward pull on the frame 18 as a whole will be precisely longitudinal.

It is to be noted at this point that, as previously stated, full tightening of each anchor bolt 42 in the threaded bore 46 of the associated pivot shaft 40 will serve to place the bolt shank 44 under tension and draw the entire frame 18 downwardly so that the arcuate edges 64 of the side plates 20 and 22 will be drawn firmly against the upper surface of the base member 12 of the girder form. However, this fact does not prevent the frame 18 from yielding to the powerful directional forces which are applied thereto incident to tightening of the cables 14. Furthermore, because the distance from the axis of the pivot shaft 40 to the upper surface of the base member 12 of the girder form is not large, and the angular deviation of the longitudinal axis of the frame 18 from the angle at which it initially was installed upon the base member is seldom greater than a few minutes of arc, i.e., less than one degree, a slight amount of slippage of the arcuate edges 62 of the side plates 20 and 22 may take place with no damage to the base member inasmuch as the center of curvature of the arcuate edges 64 coincides with the centers of the holes 58 in the side plates 20 and 22. More often than not, if the workman who installs the anchor devices 16 takes into consideration or estimates the ultimate angle which the frames 18 will assume when the cables 14 are tensioned, little or no slippage of the arcuate edges 62 on the base member will take place. Where a skilled workman is concerned, his judgment should be such that when the cables are tensioned, the only movement of the frames 18 that will occur is one which is within the molecular limits of the plywood base member 12 to withstand without slippage. If, on the other hand, the workman errs appreciably in his judgment, the upward pull of the cables 14 at the time they are tensioned will shift the frames 18 to their position of equilibrium as previously stated, regardless of the amount of tightening which has previously been applied to the anchor bolts 42.

Alternatively, a method of installation may be resorted to wherein the anchor bolts 42 are not tightened so that no tension is applied to the shank portions 44 thereof. In such an instance, the arcuate end edges 62 of the frame side plates 20 and 22 will remain free of the upper surface of the base member 12 of the girder form so that free floating movement of the frames 18 may take place under the influence of upward cable pressure on the rollers 38. When such an installation is made, adequate strength of the swivel joints is maintained inasmuch as the length of the anchor bolts 42 is such that the shank portions 44 will project completely through the diametric bores 46 in the pivot shafts 40 for full threaded engagement therewith.

The invention is not to be limited to the exact arrangement of parts shown in the accompanying drawings or described in this specification as various changes in the details of construction may be resorted to without departing from the spirit or scope of the invention. Therefore, only insofar as the invention is particularly pointed out in the accompanying claim is the same to be limited.

Having thus described the invention what we claim as new and desire to secure by letters patent is:

1. In a concrete girder form, a horizontal base member upon which wet concrete is adapted to be poured in the production of a prestressed concrete girder. a plurality of tensioning cables extending longitudinally through said form, and a plurality of hold-down anchor devices mounted on said base member for assimilating the upward thrust of the cables when the latter are tensioned, each device comprising an elongated upstanding frame including a pair of spaced apart parallel side plates, a plurality of transversely extending cable holddown members extending across said side plates and beneath which the cables extend, a pivot shaft projecting across said side plates in close proximity to the lower ends thereof, said pivot shaft having circular end faces which slidingly engage the inside faces of the end plates, trunnions projecting centrally and outwardly from said circular end faces and extending through holes which are provided in the side plates, said pivot shaft being formed with a diametrically extending bore therethrough, the lower ends of said side plates overgree of pressure.

Disclaimer 3,854,258.-J0seph J. OoZado, Addison, and N orman G. Turner, Lake Blufif, I11. ARTICULATED HOLD-DO'WN ANCHOR DEVICE FOR THE EMBEDDED CABLES OF A PRESTRESSED CON- CRETE GIRDER. Patent dated Dec. 17, 1974. Disclaimer filed Mar. 21, 197 5, by the assignee, Supem'ow Gonowete Acoessom'es, Inc. Hereby enters this disclaimer to claim 1 of said patent.

[Oyficz'al Gazette June 24, 1975.] 

1. In a concrete girder form, a horizontal base member upon which wet concrete is adapted to be poured in the production of a prestressed concrete girder, a plurality of tensioning cables extending longitudinally through said form, and a plurality of hold-down anchor devices mounted on said base member for assimilating the upward thrust of the cables when the latter are tensioned, each device comprising an elongated upstanding frame including a pair of spaced apart parallel side plates, a plurality of transversely extending cable hold-down members extending across said side plates and beneath which the cables extend, a pivot shaft projecting across said side plates in close proximity to the lower ends thereof, said pivot shaft having circular end faces which slidingly engage the inside faces of the end plates, trunnions projecting centrally and outwardly from said circular end faces and extending through holes which are provided in the side plates, said pivot shaft being formed with a diametrically extending bore therethrough, the lower ends of said side plates overhanging said pivot shaft, extending a slight distance therebelow, and being formed with arcuate lower edges which are concentric with the axis of the pivot shaft, and an anchor bolt extending upwardly through said base member of the girder form and threadedly received in and projecting completely through said diametrically extending bore and effective under tension to draw the pivot shaft downwardly and thus cause the arcuate edges of the side plates to move into sliding engagement with the upper surface of the base plate when said anchor plate is tightened with an appreciable degree of pressure. 